1. Field of the Invention
The present invention relates generally to moving target indicator (MTI) radars; and more particularly relates to an improved system and method for cancelling echoes from stationary and slowly moving objects.
2. Description of the Prior Art
Moving target indication (MTI) radar is provided to reject signals or echoes from stationary and slowly moving objects, such as terrain, foliage, or surface vehicles; and to pass echoes from moving objects such as aircraft. The radar receivers may utilize digital filters to suppress such echoes; and these filters are generally described as moving target indicators.
Typically, the undesired echoes possess a velocity spectrum, created by the scanning of the radar antenna. To achieve clutter cancellation with typical radar parameters the modulation created by scanning renders it necessary for the MTI to process the last four echoes from each point in range. The processing of these last four echoes is known as four pulse cancellation.
The MTI is blind to aircraft whose radial velocity is such that the aircraft moves an integer number of half-wavelengths between pulses. To eliminate this deficiency, the interpulse period is usually varied in a defined pattern while the antenna beam is scanning past the aircraft. In order to avoid a serious degradation of clutter attenuation utilizing the variable interpulse period (VIP), the weight or multiplying factor for each of the four pulses should be varied as a function of the three interpulse periods between the four pulses. Such weighting is to compensate for the variations in interpulse periods so that the attenuation of low velocity echoes is not degraded. Such a variation of the interpulse period, together with the weighting of the pulses is described in U.S. Pat. No. 3,560,972, together with the manner in which appropriate weights should be selected.
U.S. Pat. No. 3,566,402 describes the advantages of selecting interpulse periods in an exponential sequence to minimize the number of weighting parameters. Sequences of such interpulse period choices as disclosed in said U.S. patent are described as "exponential sawtooth" and "exponential tent" patterns. The exponential single sawtooth pattern utilizes interpulse periods that increase from the smallest to largest period in sequence or which decrease from largest to smallest. The exponential tent pattern utilizes alternate interpulse period choices from the smallest to largest; and then utilizes the interleaved remaining choices from the largest to the smallest. These types of interpulse period sequences improved the width of the clutter notch.
Although, both the systems disclosed in the U.S. Pat. No. 3,566,402 and U.S. Pat. No. 3,560,972 employ apparatus for improving the clutter notch by applying binary weighted factors to a sequence of received echo pulses from a target; and also utilize unique patterns of interpulse period choices which minimize the number of weights which must be employed, it is desirable to provide an interpulse sequence that provides still less dip insensitivity at the first blind speed, and which is more easily filtered in the high voltage power supply of the transmitter.
It is also desirable to provide an MTI radar that provides all of the advantages of a variable interpulse period and weighting as described in the aforementioned patents without the necessity of utilizing multipliers; also it is advantageous to reduce the number of bits used, which saves in storage and arithmetic hardware. Also, it is desirable to utilize only three variable weights as a function of the interpulse periods rather than four, which further saves in hardware costs.
Further, it is desirable that the variable components can be limited to one-bit binary fractions which not only further saves directly in the hardware of the system, but increases the speed of the arithmetic operation, which often results in substantial indirect savings. Finally, it is desirable that such a system utilize a single hardware design that is compatible with a number of different interpulse period sequences.